1. Field of the Invention
This invention relates to a voltage to current converter and transmitter for high output impedance transducers and, more particularly, to a so-called two-wire system for converting and transmitting the output of a high impedance transducer such as a glass-electrode pH determining system.
2. Description of the Prior Art
While prior art two-wire transmitters have been generally satisfactory, they have not been useful with certain high output impedance transducers, such as glass-electrode pH measuring systems. In order to understand the reason why prior art two-wire systems are not satisfactory, it is helpful to understand the glass-electrode pH measuring system. As will be appreciated by those skilled in the art, pH is a measure of the effective acidity or alkalinity of a solution. Pure water itself dissociates to a slight degree to form hydrogen ions and hydroxyl ions in equal concentration. The extent of this dissociation is expressed as the ion product constant of water, the value of which for pure water is 10.sup.-.sup.7 mols per liter. pH is the negative logarithm of the ion product; thus, neutral water as a pH of 7. Since the pH expression is logarithmic, there is a ten-fold change in hydrogen ion concentration per unit change in pH. The pH scale is normally considered to have a range between a pH of 0 and a pH of 14, although theoretically it is possible to have pH values beyond these limits.
The acid-base properties of an aqueous solution can be measured in a number of different ways. One of the most common, and the one to which this invention particularly relates, is the so-called glass-electrode method of measuring pH. In the glass-electrode method, two electrodes are in contact with a solution whose pH is to be measured.
One electrode is referred to as the glass-electrode and the other is known as the reference electrode. The glass-electrode is basically a glass tube closed at its bottom by a membrane of special pH sensitive glass. A buffered chloride solution, or other suitable solution, fills the tube, at least partially, so that it is in contact with the inner wall of the glass membrane. A wire, such as a silver wire coated with silver chloride, is immersed in the buffered chloride solution and forms the internal element of the glass-electrode. This wire is connected to an amplifier which converts a voltage signal generated by the glass-electrode cell whose magnitude is a function of solution pH to a current signal and transmits the current signal to a remotely located utilization device. The amplifier advantageously is located in close proximity to the electrode and is coupled thereto by a short length of insulated cable.
The potential of the glass-electrode, alone, cannot be measured so the second element, a reference electrode, is inserted in the solution. While the basic purpose of the reference electrode is to complete the electrical circuit, it must also provide a potential against which the varying potential of the glass-electrode can be measured. Typically, a reference electrode includes an electrolytic cell consisting of a silver wire coated with silver chloride which is in contact with a potassium chloride electrolyte. Contact with the sample solution is made through a liquid junction which provides a path for electron flow from the electrode to the sample solution. With this outward flow of electrolyte, the internal cell of the reference electrode is in contact with a solution which is unchanging in concentration and thus provides a stable reference potential.
The output impedances of the glass-electrode and the reference electrode are, respectively, 10.sup.8 ohms and 10.sup.4 ohms. Any appreciable current flow through the electrodes would cause an erroneous measurement and, in addition, temporary or permanent damage to the electrodes. For this reason, special amplifiers having a high input impedance and low bias currents have been developed such as, for example, U.S. Pat. No. Re. 27,688 issued to D. J. Soltz et al for "pH Detecting Device Using Temperature Compensating Field-Effect Transistor Differential Amplifier." While generally satisfactory, amplifiers such as the Soltz et al amplifier, do not provide high input impedance or low bias current when the design bias conditions do not prevail, such as a power failure condition, for example.
In addition, such prior art amplifiers require the bias supply power for the input amplifiers to be isolated from the power supply ground. This requirement has prevented the use of so-called two-wire systems such as that shown in U.S. Pat. No. Re. 27,596 issued to E. T. Hurd, III entitled "Two Wire MV/V Transmitter." This system (Hurd) cannot be used because of low input impedance when power is off. In addition, it is undesirable because of the need for bias power isolation for the input signal conditioning means.